This invention relates to an image generation system for use in an electrographic printer or copying machine capable of duplex printing, printing on both sides of a sheet of paper. More specifically, the present invention relates to an image generation system which can accept a description of the text and graphics to appear on the paper from a host computer system and compute the bit pattern in the bit-map RAM corresponding to the desired image.
In the process of electrographic or xerographic printing, a photoconductive member is employed to record an image. The photoconductive member, which may be in the form of a belt or a drum, is charged to a substantially uniform potential to sensitize its photosensitive surface. In the case of a copying machine, a light is projected on to an original document to be copied. Through the use of lenses, mirrors, and various other optical components, the charged portion of the photoconductive surface is exposed to a reflected light image of an original document to be reproduced. The light image is recorded as an electrostatic latent image on the photoconductive member. This latent image corresponds to the informational areas contained on the original document.
In the case of an electrographic printer connected to a computer, a similar process is used to record information on the photoconductive member. The charged portion of the photoconductive surface is exposed to a light image produced by an optical print head. The precise shape of the light image is controlled by input signals from the computer. For example, a laser or an LED array may be used as an optical print head which receives input signals from the computer to illuminate the photoconductive member with a light image of a particular shape. Here too, an electrostatic latent image corresponding to the desired informational areas is recorded on the photoconductive member. The control of the LED array determines not only what is printed but whether the printing is from top to bottom or bottom to top, or any other predetermined pattern.
As used herein, the term "electrographic printing apparatus" and the like refer to both electrographic printers and copiers. As used herein, the term "printing engine" refers to the apparatus for printing on paper.
After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material or toner into contact with it. The developer material is attracted to the electrostatic latent image and forms a powder image on the photoconductive member corresponding to the electrostatic latent image. The powder image is subsequently transferred to a sheet of recording medium, such as a sheet of paper, in a transfer region. Thereafter, the powder image is permanently affixed to this sheet in image configuration by a variety of methods, such as by fusing.
The above-mentioned operations may be carried out by arranging a number of stations in sequence about the photoconductive member. Thus, the photoconductive member is usually surrounded in sequence by a charging station to charge the photoconductive member, an imaging station to form an electrostatic latent image on the photoconductive member, a developing station to develop the electrostatic latent image on the photoconductive member, and a transfer station to transfer the developed image from this photoconductive member to the sheet of recording medium. A discharging station and a cleaning station are also arranged about the photoconductive member to ready it for use again.
An example of such an electrographic printing apparatus is disclosed in allowed application Ser. No. 700,813, filed Feb. 11, 1985, now U.S. Pat. No. 4,664,507. The electrographic printing apparatus described in that patent appplication employs a photoconductive belt assembly in the form of a disposable cassette which is the subject of allowed application Ser. No. 718,947, filed Apr. 2, 1985, now U.S. Pat. No. 4,657,369. The electrographic printing apparatus described in application Ser. No. 700,813 also employs the combined developing and cleaning unit which is the subject of allowed application Ser. No. 718,946, filed Apr. 2, 1985, now U.S. Pat. No. 4,639,116. All of the aforementioned patent applications are assigned to the present assignee and all are incorporated herein by reference.
The electrographic printing apparatus described in application Ser. No. 700,813 has a simplified paper path permitting access from the top of the machine. In that electrographic printing apparatus, the cassette containing the photoconductive belt is mounted vertically within the machine and a latent image is developed on the underside of the copy sheet as it passes over and comes in contact with the top of the photoconductive belt assembly.
The electrographic printing apparatus described in that application requires two rotations of the photoconductive belt per copy produced. In actual practice, it is capable of producing about 12 copies per minute. During the first rotation of the photoconductive belt, the belt is uniformly charged and a latent image is generated by means of an optical print head on the surface of the photoconductive belt. The latent image thus formed is developed by the deposition of toner from a combined developer/cleaning unit operating in the develop mode. The belt then enters the transfer region wherein the developed image is transferred to the underside of the paper or other copy material. In the transfer region, a transfer unit generates an electrical field which attracts the toner from the photoconductive belt to the underside of the paper. This completes the first rotation of the belt as the paper travels to a fuser unit and is discharged into the output tray.
During the next revolution of the belt, the belt is prepared for making the next copy. The main charging unit and the optical print head are disabled while an erase lamp is activated and the developer/cleaner unit is switched to the clean mode. Thus, as the belt continues to rotate following image transfer, the photoconductive belt is discharged by an erase lamp and the excess toner is removed using a conventional electrostatic process by the developer/cleaner unit. The belt is thereby readied for printing on the next page.
The electrographic printing apparatus described in application Ser. No. 700,813 is to provide a machine which reliable over an extended period of use and which is easily serviceable. To accomplish this, the machine is provided with modular units which are easily removed and replaced at specified time intervals or when they malfunction. In addition, the machine is provided with a simplified paper path whereby the paper always travels along a substantially planar path located near the top of the machine. The paper is imprinted on its underside as it travels along this paper path and passes across the top of the vertically mounted photoconductive belt.
By providing this "straight-through" paper path, the number of paper jams is considerably reduced. In the event a paper jam does occur, the lid of the machine can be opened and the jammed paper can be readily accessed from the top of the machine. This is in contrast to prior art machines wherein paper jams can only be accessed from the side of the machine. Additionally, because the paper is imprinted on its underside, and because the machine has a "straight-through" paper path, the paper is ejected into the output tray face down. Thus, the paper is automatically collated as it is being printed and ejected.
The electrographic printing apparatus disclosed in application Ser. No. 700,813 does not perform duplex printing, i.e., the ability to print on both sides of a sheet of paper. Duplex printing is important in many applications, for example, in printing a book or in so-called "desktop publishing" applications.
Duplex printing utilizes an image generation system which can print one side of a paper in one direction and the other side in the opposite direction in order to simplify the mechanical handling of the paper in turning it over.
The image generation system in the application Ser. No. 700,813 does not include the capability of printing in opposite directions.
In addition, the prior art image generation system (IGS) is limited to a print head of 240 dots per inch (dpi) and uses 1 MB of bit-map RAM to build an 81/2 inch/by 14 inch paper at 240 dpi.
There is a need for a high speed duplex printer which can operate with 240 dpi and higher dot densities.